The present invention relates to a method and apparatus for relieving thermal stresses caused by temperature changes in glass lined metal vessels.
Glass, or enamel, internally coated tanks and process equipment have been in wide industrial use for many years where smooth, corrosion resistant, internal surfaces are desired or required. Such equipment is particularly adapted to use in extraction, separation and distillation processes in various chemical, food, and beverage processing industries, where it is in extensive use as reactors, tanks and many types of storage, aging and mixing equipment.
The contour of pressure vessels is generally rounded with ellipsoidal or hemispherical ends to provide better stress distribution. In glass lined vessels the rounded shape also acts to minimize thermal stresses which develop between the glass lining and the metal substrate during temperature changes. Additionally, a rounded internal contour allows the vessel to be easily emptied, cleaned or flushed. Thus, glass lined vessels typically are cylindrical with rounded tops and bottoms. When such vessels are installed in a standing or vertical position, structural support such as legs or a support skirt are required. A skirt is an annular projection that typically extends downward from the side or bottom portion of the vessel beyond the bottom contour and has sufficient dimensional stability to support the vessel in a vertical position.
Glass lined vessels supported by skirts are frequently subject to failure at the bottom knuckle radius of the vessel when the vessel is heated or cooled. The knuckle radius, or transition knuckle, is the area of the vessel adjacent the junction of the contoured, usually spherical or ellipsoidal, bottom and the cylindrical sidewall. Temperature changes from ambient occur when the vessel itself is heated or cooled, or when hot or cold material is introduced into the vessesl. For example, when the vessel is heated, or when heated materials are added to the vessel, the vessel diameter expands. The skirt, especially the bottom portion, (the portion most remote from the vessel), remains at ambient temperature for some time and then slowly warms, but only to a temperature substantially below that of the vessel. Thus, the skirt introduces a bending moment in a knuckle area of the vessel caused by the relative movements of the skirt and vessel wall. This moment when superimposed upon other thermal or mechanically induced stresses on the vessel may be of sufficient magnitude to cause failure of the vessel lining. Correspondingly, a bending moment in the same area is created when the vessel is cooled, or cooled materials are added to the vessel. In such case, the vessel wall contracts while the skirt tends to remain in a relatively expanded state.
The present invention seeks to substantially reduce thermally induced strains in the knuckle radius area of a vessel when the vessel undergoes a temperature change.